Hybrid hard disk drive (HDD) as one of storage devices include one or more rotating magnetic disks combined with nonvolatile solid-state (e.g., flash) memory. Generally, a hybrid HDD has both the capacity of a conventional HDD and the ability to access data as quickly as a solid-state drive, and for this reason hybrid HDDs are expected to be commonly used in laptop computers.
However, a drawback of the NAND-based memory cells in solid-state drives and hybrid HDDs is that such memory cells are only operable for a limited number of program/erase cycles. Furthermore, at higher temperatures, data retention of NAND memory cells is greatly reduced. For example, when single-level cell (SLC) NAND memory cells are elevated to a temperature greater than 70 to 80° C., even short-term retention of data of these NAND cells may become unreliable, and they cannot be considered a dependable nonvolatile data storage medium. This is because data stored in NAND cells may be lost at anytime under such conditions. For a hybrid HDD that includes multi-level cell (MLC) NAND memory, reliability is even more of an issue, since MLC cell reliability may be compromised after fewer program/erase cycles and at lower temperatures than SLC cells.
Because reads from and writes to NAND memory are much faster than reads from and writes to a magnetic disk, a significant advantage of hybrid HDDs over conventional HDDs is that the NAND memory included in a hybrid HDD can be used as a large read and write cache that reduces the frequency of relatively time-consuming disk accesses. In fact, with enough NAND memory, the performance of a hybrid HDD can approach that of a solid-state drive, since disk accesses are so infrequent. But as NAND memory cells become less reliable, this improved performance is generally lost, since the NAND memory cells of the hybrid HDD can no longer be used to reliably store for an extended period of time. NAND memory cells may become less reliable through wear, or may be considered inherently unreliable if the maximum temperature at which the memory cells can reliably retain data is too low to store dirty data (that is, data for which an up-to-date copy does not exist on the Disk) in the NAND. In either case, such NAND memory cells are generally unsuitable for use as a write cache that quickly receives and stores data for subsequent writing to disk, a principle advantage of a hybrid HDD. So while hybrid HDDs can incorporate some of the advantages of solid-state drives, in certain situations these advantages may be lost.